DC-DC switching power converters are generally configured to provide regulated DC (direct current) output voltage or output current to load based on un-regulated DC source. For instance, in most automobile applications, un-regulated DC voltage of about 12 volts is provided as power source. DC-DC switching power converters may be applied to receive the un-regulated DC voltage of about 12 volts to provide regulated output voltage or output current to drive various electronic circuits (e.g. instrument panels, lighting devices, motor controllers, radio/audio devices etc.) in the automotive vehicles. The regulated output voltage can be lower, higher or equal to the un-regulated DC voltage.
Commonly used control methods for controlling switching power converters may include average current control method, peak current control method and constant on time (COT) control method etc. Average current control method and peak current control method permit accurate detection of the output voltage or output current of the switching power converters to provide feedback information indicative of the output voltage or output current to a control loop. However, the control loop generally requires complicated compensation circuits to maintain the stability and an appropriate band width of the control loop. Moreover, switching power converters that employ the average current control method and peak current control method may have bad load transient performance. Switching power converters employing the COT control method may not require the compensation circuits, which simplifies the design and reduces the cost, but the switching frequency of COT controlled switching power converters tends to change with the input voltage or load current of the switching power converters. For obtaining a stable switching frequency, usually a phase-locked loop module may need to be designed for COT controlled switching power converters. However, the phase-locked loop module requires a clock signal of higher frequency than the switching frequency and the phase-locked loop itself requires compensation, which is hard to design. Therefore, there exists a need to design a control circuit for the COT controlled switching power converters to maintain the switching frequency of the COT controlled switching power converters substantially constant.